Questions about whether COVID-19 leaked from a Chinese laboratory have cast a chill over American virus research, drying up funding for scientists who collect or alter dangerous pathogens and intensifying a debate over those practices.

The pullback has transformed one of the most highly charged fields of medical science. While some believe such experiments could fend off the next pandemic, others worry that they are more likely to start one.

At Pennsylvania State University, a proposal to infect ferrets with a mutant bird flu virus passed the federal government’s most rigorous biosafety review only to be rebuffed by the National Institutes of Health. Troy Sutton, the scientist behind the studies, said health officials referred to the public controversy over the lab leak theory in advising him to pursue different experiments.